Kingfisher hovering

The Belted Kingfisher may be the largest bird in North American capable of (true) hovering.

Hovering flight is energetically expensive and only small birds can manage it. To stay in one spot in still air, the bird must generate lift solely by flapping its wings. Regular flight, where lift results from the bird’s forward motion through the air, requires only half the power of hovering. Large birds (with their lower ratio of power output to mass) just cannot sustain the energy output needed to hover. Sometimes a large bird appears to be hovering, but is actually holding a fixed ground position by flying into a strong wind.

I regularly watch hummingbirds hover—they are the masters of it. I knew that the significantly heavier kingfisher occasionally hovered when fishing, yet I had only ever seen it hunt from a perch. Hovering is exceedingly hard work; why do it when it isn’t needed?

Sunday, I watched a female Belted Kingfisher hover.

There was no wind, but the bird stayed fixed in space over the water. To enable details to be seen, separate images were first combined and then displaced—time moves like text on a page. One interesting detail is the bird’s use of alulae (Latin, for winglet). The kingfisher only deploys them on the downstroke, but retracts them during the upstroke. Unlike a hummingbird, the kingfisher is incapable of generating lift on the upstroke, only the downstroke.

Ten pictures in two seconds. Wings are folded during upstroke and spread during downstroke. The alulae are the extensions midway along the leading edge of the wing. They perform the same function as slats on an aircraft and allow the wing to achieve a higher than normal angle of attack. This maintains lift without resulting in a stall.

A detail of the final image is included to make it easy to spot the alula along the leading edge of the right wing.

The following composite does not show a kingfisher hovering but landing on a tree branch. The bird flies upwards using gravity to slow itself (images get closer together). At the last moment, when it has a particularly high angle of attack, the alulae are deployed.